Recently, along with a rapid progress of an information transmission technology, various information transmissions become possible. From among these information transmissions, a wireless information transmission is very superior and is widely utilized from a standpoint of its usability.
Means for transmitting information through the wireless transmission includes various electronic equipments such as a cellular phone, PDA (Personal Digital Assistance), a wireless LAN (Local Area Network), a broadcasting wave, an automotive vehicular radar, ETC (Electronic Toll Connection System) mounted device, and so forth.
On the other hand, along with a popularization of the wireless information transmission, an electromagnetic wave emitted from the equipment used in the wireless information transmission is invaded into a building from an opening formed on the building to serve as an electromagnetic noise for the other pieces of equipment. Hence, an electromagnetic wave absorbing plate having a transparency and which can effectively absorb the electromagnetic wave becomes demanded.
A wireless LAN (Local Area Network), from among the wireless information transmissions, requires no LAN construction in the indoor (cable wiring construction and so forth). Hence, the wireless LAN contributes largely on a cost reduction and on easiness in handling in an office and in a general home.
However, the wireless LAN gives many problems such as a reduction in a LAN speed due to an influence of a reflective plate (desk, rocker, chair, and so forth) in the case of an indoor use, a tap due to an electric wave leakage toward an outdoor, an ill effect due to an electric wave interference (due to four channels in 2.45 GHz range) in the building or between buildings, and generations of external unauthorized access, spoofing, and so forth. As a countermeasure of such problems as described above, such a countermeasure as an exchange of issurer certificates between communication terminals such as PC and a server, a data encryption, or a regular automatic modification of an encrypt key, an ID, and a password involves a high possibility of a third person's decipher.
Therefore, a transparent electromagnetic wave absorbing plate is required to be attached onto an indoor room partition and an opening on the building or between buildings. Furthermore, in an ETC lane such as a toll gate in an express way, in order to prevent an erroneous operation due to a reflective wave from a vehicle on a general traffic lane, the transparent electromagnetic wave absorbing plate is required as a partitioning wall between an ETC lane and a general traffic lane and as a partitioning wall between mutual ETC lanes.
It is generally known that, as the electromagnetic wave absorbing plate which absorbs the electromagnetic wave, for example, two layers of laminated ITO (Indium-Tin Oxide) conductive films laminated in a transparent film, one layer being used for an electromagnetic wave absorbing material and the other layer being used for an electromagnetic wave reflecting material, and an interval between the two layers can be adjusted to be ¼ wavelength of the electromagnetic wave to be absorbed (refer to a Patent Literature 1).
An electromagnetic wave absorbing material in which the interval between the electromagnetic wave absorbing material and the electromagnetic wave reflecting material is ¼ the absorption frequency has resulted in a thickness in a range from 10 cm to 1 m or thicker, in a case where a frequency band to be absorbed by this electromagnetic wave absorbing substance is a VHF band. In order to thin this thickness range, another type of electromagnetic wave absorbing plate has been proposed in which an insulating substrate on which conductive coatings are formed in stripe shapes or in lattice shapes is interposed between the electromagnetic wave absorbing plate and electromagnetic wave reflecting plate to increase an effective permeability between the electromagnetic wave absorbing material and the electric wave reflecting material (refer to a Patent Literature 2).
Furthermore, a technique to obtain an electromagnetic wave absorbing effect utilizing a phase difference between the electromagnetic wave reflected on a surface of an impedance layer and the electromagnetic wave reflected on a surface of a dielectric layer has been disclosed (refer to a Patent Literature 3).
Patent Literature 1: Japanese Patent Application Publication (Tokkai) No. 2001-44750;
Patent Literature 2: Japanese Patent Application Publication (Tokkai) No. Heisei 10-275998; and
Patent Literature 3: Japanese Patent Application Publication (Tokkai) No. 2003-8279.
In a case where the electromagnetic wave absorbing material in which the interval of distance between the electromagnetic wave absorbing material and the electromagnetic wave reflecting material is ¼ the wavelength of the electromagnetic wave to be absorbed, a thickness of the electromagnetic wave absorbing material requires ¼ the wavelength of the electromagnetic wave to be absorbed and indicates about 31 mm.
In order to attach this electromagnetic wave absorbing material to the partition of a room and a floor passage or of a window of a building, such a problem that the thickness of the material itself is too thick. In addition, the other type of electromagnetic wave absorbing material in which the insulating material on which conductive coatings are formed in the strip shapes or in the lattice shapes is arranged, the thickness of the electromagnetic wave absorbing plate can be decreased. However, its structure is complex and a mass production is difficult. Furthermore, in a case where the impedance layer is installed only at one side of the transparent electromagnetic wave material, the electromagnetic waves cannot be bi-directionally absorbed.